The medial wall of the orbit is parallel to the antero-posterior axis of the eye. On the other hand the lateral wall forms 45 degrees with the medial wall. So that the axis of the orbit will make 22.5 degrees with antero-posterior axis of the eye.

The muscles moving the eye:-

The eye moves by 6 EOM; 4 recti and 2 obliques.

The 4 recti muscles:

They originate from " the common annular tendon of Zinn " at the orbital apex around the optic nerve canal, then pass forwards forming a cone to be inserted in the anterior half of sclera 5 - 7 mm. posterior to the limbus.

N.B. The S.R. and I.R. are parallel to the orbital axis, so that they make 23 degrees angle with the antero-posterior axis of the eye.

The 2 oblique muscles:-

The S.O. arises from the orbital apex, runs forwards, then bends on a cartilaginous trochlea just behind the upper medial angle of the orbital margin. Then it passes backwards and laterally to be inserted on the upper surface of the globe behind the equator.

The I.O. originates from the bone just lateral to the opening of the NLD. Then it passes backwards and laterally to be inserted in the lower surface of the eye behind the equator.

N.B. Both obliques form a 51 degrees angle with the antero-posterior axis of the eye.

Actions of EOM:-

- M.R.: Pure adduction.

- L.R.: Pure abduction

- S.R.: In the primary position it causes elevation (main action) plus intortion and adduction.

- I.R.: In the primary position it causes depression (main action) plus extortion and adduction.

- S.O.: The main action is depression of the adducted eye during reading. In the primary position, it causes intortion (main action) plus depression and abduction.

- I.O.: The main action is elevation of the adducted eye. In the primary position it causes extortion (main action) plus elevation and abduction.

Nerve supply of EOM:

All are supplied from oculomotor (III) except LR6 (abducent) and SO4 (trochlear).

Movements of the eye:-

* Around the horizontal axis ---> elevation and depression.

* Around the vertical axis ---> adduction and abduction.

* Around the antero-posterior axis ---> intortion and extortion (referring to the 12 o’clock position).

Laws of eye movements:-

* The law of reciprocal innervation: When one muscle is stimulated, its antagonist receives equal inhibition.

* Stimulation of one muscle in one eye is accompanied by equal stimulation of its yoke muscle in the other eye. For example stimulation of the MR or right eye is accompanied by equal stimulation of LR of left eye, so that both eyes move equally to the left side.

Axes of the eye:-

1. The optic axis:-

It is the imaginary line passing through the optical centres of the refractive media of the eye (centres of cornea and lens) and the nodal point.

2. The visual axis:-

It the axis passing from the fovea, nodal point and point of fixation. so that we look by our visual axes not by the optic axes. The visual axis passes through the cornea nasal to the optic axis.

Both visual and optic axes meet at the nodal point and form a 5 degrees angle between each other in emmetropes. this angle is called "angle alpha". In hypermetropes it is more than 5 degrees causing apparent divergent squint. In myopes it is less than 5 degrees causing apparent convergent squint.

Binocular vision:-

Definition:-

It is the ability of the brain to fuse the images of both eyes to make a single mental impression.

Grades of binocular vision:-

Grade I: Simultaneous perception. It means that the brain is able to perceive images from both eyes in the same time, not from one eye at a time.

Grade II: Fusion. This is to form a single mental impression. Images should be nearly similar and coming from corresponding retinal points.

Grade III: Steriopsis. It means that the brain could interpret the slight difference between the images of both eyes to feel the depth of image (i.e; the depth of perception).

Requirements of binocular vision:-

1. Sensory requirements: Both retinal images should be equal in shape, clarity, size and colours in order to be fused by the brain. This requires that both eyes should have equal refraction, clear media, functioning retina and intact visual pathway.

2. Motor requirements: Normal EOM with intact innervation.

3. Cortical requirements: Good binocular centre.

If these requirement are available binocular vision develops spontaneously early in life.

Development of binocular vision:

- Macular vision starts at the age of 6 weeks.

- Binocular fixation starts at the age of 6 months.

- Binocular vision development is completed and becomes stable by the age of 6 years.

Testing of binocular vision:

Many tests are used to evaluate the degree of binocular vision. The most important 2 tests are:

1. Worth 4 dots test.

2. The major amblyoscope (synaptophore).

Accommodative convergence / accommodation ratio (ac / a):-

* Normally accommodation occurs hand with hand with convergence (ac / a = 1). This is because both serve the same purpose (near vision), and accordingly, they are parts of the same reflex (the near vision reflex).

* With errors of refraction, the amount of accommodation either decreases in myopia leading to true divergent squint, or becomes excessive in hypermetropia leading to convergent squint. So that:

1. Myopia can cause divergent squint, either latent or concomitant.

2. Hypermetropia can cause convergent squint, either latent or concomitant.

Squint:-

Squint is defined as the abnormal relationship between the visual axes of both eyes.

Orthophoria (the normal relation between the visual axes of both eyes):-

Orthophoria = parallel visual axes with no tendency to deviate even when binocular vision is disturbed.

Types of squint:-

I. Apparent squint.

II. True squint: - Latent squint.

- Manifest squint: - Concomitant squint.

- Paralytic squint.

Assessment of a case of squint:

Symptoms and history taking:

The most important symptoms are visual asthenopia (latent squint), cosmetic disfigurement (concomitant and apparent squint) and acute diplopia (paralytic squint).

The patient should be asked about the age of onset, similar family conditions and history of trauma.

Examination of a case of squint:

After complete ophthalmic examination to exclude any organic disease in the eye, the following tests should be done for each case of squint:

1. Retinoscopy under complete cycloplegia.

2. Measuring visual acuity after correction of the error (decreased if there is amblyopia or organic lesions).

3. Assessment of the primary angle of deviation (PAD): The patient fixates at a point source of light about 50 cm. away without covering any eye. Normally the image of the source of light on his cornea is centralized in relation to the pupil in both eyes. If one eye is squinted the direction will be easily known and the angle of deviation will be roughly measured as following:

- Image of source of light at the pupillary margin = 15 degrees.

- Image of source of light between pupil and limbus = 30 degrees.

- Image of source of light at limbus = 45 degrees.

- Image of source of light on the conjunctiva = 60 degrees.

4. Assessment of the secondary angle of deviation (SAD) by the cover test (see later).

5. EOM motility assessment as following:

- LR ---> look laterally.

- MR ---> look medially.

- SR ---> look up and laterally.

- IR ---> look down and laterally.

- SO ---> look down and medially.

- IO ---> look up and medially.

NB. Each eye is examine separately then both eyes together.

6. Other tests depending on the type of squint.

1. APPARENT SQUINT

It is the condition in which the patient looks squinted in spite of being orthophoric (having parallel visual axes with no tendency to deviate even when binocular vision is disturbed).

So that the complaint of the patient is only cosmetic.

Types and causes:-

I. Abnormal angle alpha:

1. High axial myopia with small angle alpha lead to apparent convergent squint.

1. High axial hypermetropia with large angle alpha lead to apparent divergent squint.

- Refraction may be either normal (in cases of ankyloblepharon and epicanthus), myopic or hypermetropic.

- Visual acuity is normal after correction of errors.

- Corneal reflex test shows no deviation.

- Cover test shows no deviation under the cover as in normal subjects.

- No limitation of EOM movements.

Treatment:-

1. Cosmetic correction is done if possible for cases of epicanthus and lateral ankyloblepharon.

2. Correction of error.

2. LATENT SQUINT (HETEROPHORIA)

Definition:-

It is the tendency of both eyes to dissociate from each other when binocular vision is disturbed.

This means that there is a desire to dissociate, but this desire is checked by the binocular vision centre (compensated heterophoria). Deviation (decompensation) occurs only when binocular vision is disturbed.

Causes of decompensation of phoria are:

* Covering one eye (the cover test).

* Making each eye see an object different from the object seen by the other eye as in Maddox rod an Maddox wing tests.

* Mental and physical fatigue, anemia and in fevers.

Types of heterophoria:-

- Esophoria = tendency of inward deviation.

- Exophoria = tendency of outward deviation.

- Hyperphoria = tendency of upward deviation of one eye (Rt or Lt).

- Incyclophoria = tendency of the 12 o'clock point for inward deviation.

- Excyclophoria = tendency of the 12 o'clock point for outward deviation.

Causes of phoria:-

I. Errors of refraction:-

1. Hypermetropia causes esophoria due to over accommodation leading to over convergence leading to esophoria.

2. Myopia causes exophoria due to less accommodation leading to less convergence leading to exophoria.

3. Oblique astigmatism may cause cyclophoria.

II. Physiological causes:-

1. Prolonged neglect of near work causes weak convergence, so that after long holidays students complain from exophoria during reading.

2. Age: Esophoria is common in children while exophoria in adults.

3. Prolonged uniocular work as microscopes may cause latent squint.

III. Weaknessof one or more of EOM.

Clinical picture:-

Symptoms:

Heterophoria is very common and it is usually asymptomatic (compensated). When it is decompensated the patient will complain of visual asthenopia (due to muscular asthenopia):

1. Eye strain:

- Ocular pain, headache and redness starting few hours after starting near vision work and disappear on closing the eyes.

- Recurrent styes and blepharitis.

2. Transient deviation of both eyes from each other will cause running letters or running lines. Sudden intermittent diplopia may occur leading to vertigo and vomiting especially in cases of vertical phoria.

3. Difficulty in changing focus from near to far or vice versa.

4. Squint by night or during fevers.

5. Patients of severe exophoria may suffer from photophobia that is relieved by closure of one eye.

Signs:

1. Refraction may show: - Myopia in cases of exophoria.

- Hypermetropia in cases of esophoria.

2. Visual acuity is normal after correction of errors.

3. No PAD.

4. SAD is present as shown by cover test:

- The patient looks at a point source of light 50 cm. away.

- Any eye is covered to dissociate binocular vision. The covered eye will deviate under the cover.

- When the cover is removed the recovery movement will be noticed. The direction of squint is known from the direction of recovery movement.

5. No limitation of EOM movements.

6. Tests to confirm phoria and measure its degree accurately:-

a. Maddox rod test to measure phoria for far vision:

* The patient looks at a point source of light at 6 m. distance, then we place a special lens in front of one eye. This lens turns the image seen by this eye into a line, while the other eye sees a spot of light. So that binocular vision will be disturbed and deviation occurs.

* If no deviation occurs (orthophoria), the person will see the spot of light on the line. If deviation occurs (phoria) the line will be seen on one side of the spot of light to fall on a scale. So that the degree of deviation is measured directly.

* We do this while the line in the vertical position to measure horizontal phoria and another time when the line in the horizontal position to measure the vertical phoria.

b. Maddox wing test to measure phoria for near:-

* It also depends on the principle of dissociation of the two retinal images. It gives directly the direction and degree of phoria for near vision.

c. Major amblyoscope.

Complications:-

It may turn concomitant especially after fever that lasts for few days.

Treatment:-

I. If no symptoms (compensated) we give no treatment.

II. If symptomatic (decompensated) we do:

1. Correction of the error of refraction if present.

2. Training of the weak muscles by certain exercises or training prisms. The base of the prism is placed towards the direction of deviation (direction of the strong muscle).

3. Surgery is indicated in few cases with very large degrees of latent squint not responding to the above treatment.

3. CONCOMITANT SQUINT (HETEROTROPIA)

Definition:-

It is the abnormal relationship between the two visual axes, but the angle of deviation is constant in all direction of gaze.

Causes of tropia:-

a. Poor vision in one eye:

In these cases fusion can not occur because images of both eyes are not equally clear. The brain will always neglect the image of the worst eye that will eventually become amblyopic. In these cases squint is unilateral. The causes are:

- Organic lesions as nebula, cataract and retinoblastoma.

- Anisometropia (the eye with more error will be amblyopic).

b. Uncorrected errors of refraction (the commonest causes):

1. Hypermetropia causes esotropia due to over convergence associated with over accommodation.

2. Myopia causes exotropia due to under convergence associated with under accommodation.

Bilateral equal errors cause alternating squint.

c. Central lesions:

There is marked weakness of the fusional capacity of the brain. It accounts for the few cases squint in which there are no sensory or motor abnormalities.

Types of tropia:

A) According to direction:

1. Esotropia = concomitant convergent squint. It is the commonest.

2. Exotropia = concomitant divergent squint.

3. Hypertropia and hypotropia = upward or downward deviation of one eye.

4. Incyclotrpia and excyclotropia are very rare.

B) According to which eye is deviated:

1. Unilateral squint:

- One eye is deviated all the time. This is because this eye gives indistinct vision that can not be fused with the good image of the other eye.

- It occurs due to a unilateral lesion as unilateral error of refraction (anisometropia), corneal opacities, congenital cataract, ....

- Vision of the squinted eye is poor due to amblyopia, while vision of the other eye is normal.

- The condition may be complicated by eccentric fixation leading to an abnormal form of binocular vision.

2. Alternating squint:

- The patient sees equally with either eyes, so that the brain will not prefer the image of one eye over the other. One is squinted and the other eye is fixating for some time, then switching of fixation occurs and so on.

- The great majority of cases occur due to bilateral equal uncorrected errors.

- The brain will not suppress any eye, so that no amblyopia develops, so that vision of both eyes is good after correction of the error of refraction.

- This condition can be complicated by abnormal retinal correspondence (ARC) which is an abnormal form of binocular vision.

Clinical picture:-

Symptoms:

Cosmetic disfigurement (no diplopia and no asthenopia).

Signs:

At first, examination of the eye should be done for diagnosis of the etiology and exclusion of retinoblastoma which may presented only by squint.

1. Refraction: - Hypermetropia in cases of esotropia.

- Myopia in exotropia.

2. Visual acuity after correction of errors:

- Good in both eyes in alternating cases.

- Poor in the squinted eye in unilateral cases.

3. Primary angle of deviation (PAD):

- It is constant in all directions of gaze.

- It is used as a rough method of measurement of the angle of squint:

* At the edge of pupil = 15 degrees.

* Between pupil and limbus = 30 degrees.

* At limbus = 45 degrees.

* On the conjunctiva = 60 degrees.

4. Secondary angle of deviation (SAD) by the cover test:

The patient is asked to look at a point source of light 50 cm. away, then we cover the fixating eye, so that the squinting eye will fixate. This test is done for 2 purposes:

a. To differentiate between concomitant and paralytic squint:

- In concomitant squint SAD = PAD.

- In paralytic squint SAD > PAD.

b. To differentiate between unilateral and alternation squint:

- In cases of unilateral squint the covered eye will regain fixation once the cover is removed.

- In alternating cases the new state of fixation will be maintained after removal of the cover.

5. Ocular motility examination:

There is no limitation of movement (to differentiate from paralytic squint).

It is a special instrument used for diagnosis and treatment of squint. Each eye looks through a tube into an object that differs from the object seen by the other eye. According to the relation between the 2 objects we can diagnose the grade of binocular vision:

- Simultaneous perception.

- Fusion.

- Steriopsis.

Also the tubes can be moved on a protractor to measure the angle of squint accurately.

Treatment of concomitant squint:-

The aims of treatment are:

1. Improvement of visual acuity of suppressed amblyopic eyes.

2. Improvement of binocular vision.

3. Cosmetic treatment.

These goals are achieved through the following lines of treatment:

1. Etiological treatment:

1. Correction of the error of refraction by:

- Glasses above the age of 2 years.

- Long acting miotic eye drops in hypermetropic children below 2 years.

- Contact lenses for anisometropia.

2. Surgical correction of organic lesions as extraction of congenital cataract and followed by optical correction.

2. Treatment of amblyopia:

1. Amblyopia without eccentric fixation:

We cover the normal eye to obligate the brain to use the amblyopic eye.

2. Amblyopia with eccentric fixation:

We cover the amblyopic eye at first to forget the eccentric focus, then cover the normal eye.

3. Orthoptic treatment:

It is done by the synaptophore and other instruments to correct amblyopia and to increase the capacity of fusion. It is very important to build and improve fusion to prevent recurrence of squint after surgical correction.

4. Surgery:

* Indications:

a. Failure of the above measures as in cases above 6 - 7 years.

b. Big angle of squint partially improved by glasses. We do surgery only for the residual angle.

c. Cosmetic.

* Contraindications:

Eccentric fixation and ARC because the patient will suffer from diplopia after surgery.

* Techniques:

a. Recession: It is done to weaken the strong muscle. In this surgery the muscle insertion is cut then sutured to sclera few millimetres posterior to the original site.

b. Resection: It is done for strengthen the week muscle. Few millimetres from the muscle are excised near its insertion. Then the remaining part is stretched and sutured to the site of original insertion.

Each 1 mm. of muscle surgery corrects 2 - 3 degrees of the angle of deviation.

NB. Uses of atropine in concomitant squint:-

1. Pupillary dilatation for fundus examination.

2. Cycloplegia for refraction.

3. To blur the normal eye in cases of unilateral squint as a method of cover therapy.

4. PARALYTIC SQUINT

Definition:-

Squint due to paralysis of one or more of EOM.

Etiology:-

The commonest causes are increased ICT (damage of VIn) and diabetic neuropathy. Other causes of nerve lesions are:

A. Lesions of III, IV or VI nuclei:-

1. Congenital aplasia especially trochlear (IV) nerve.

2. Inflammatory lesions as encephalitis.

3. Vascular lesions as cerebral hemorrhage and thrombosis.

4. Brain stem tumours.

B. Lesions of III, IV or VI nerves:-

1. Inflammatory lesions as meningitis and orbital cellulitis.

2. Fracture base of skull.

3. Vascular lesions as cavernous sinus thrombosis and subarachnoid

hemorrhage.

4. Neoplastic lesions as orbital tumours.

C. Injury of EOM:-

1. Myasthenia gravis causing diplopia by the end of the day.

2. Traumatic injuries.

3. Thyroid exophthalmos.

Clinical picture:-

Symptoms:

Diplopia is the presenting symptom:-

- It is binocular diplopia; i.e; relieved by closure of either eyes.

- It causes headache, vertigo and vomiting.

- The image of the squinted eye is called "the false image" always blurred and projected to the direction of action of paralyzed muscle.

- the false image may be projected to the direction of paralyzed eye (uncrossed diplopia in cases of VIn palsy) or to the opposite direction (crossed diplopia in cases of IIIn palsy).

- It is maximum when the object is placed in the direction of action of paralyzed muscle and minimum in the opposite direction. This fact is well evident in diplopia chart.

- Face turn to the direction of action of paralyzed muscle to minimize diplopia (compensatory head posture).

Signs:

1. Refraction: No errors.

2. Visual acuity is usually good unless the condition is complicated by strabismic amblyopia.

3. ThePAD:-

- The eye is deviated opposite to the direction of action of paralyzed muscle.

- It is variable being maximum in the direction of action of paralyzed muscle.

4. Cover test shows that the SAD is more than the PAD (why).

5. EOM examination shows defective motility of the paralyzed muscle (the most important sign).

6. Other tests and investigations:

a. False projection test: Close the normal eye of the patient and ask him to touch your finger by his finger. His finger will be displaced to the direction of action of paralyzed muscle.

b. Diplopia chart is used to confirm diagnosis and for monitoring the prognosis of the condition.

c. Neurological examination and investigations for diagnosis of the etiology.

What is the clinical picture of VIn paralysis ?

What is the clinical picture of IIIn paralysis ?

Complications:-

1. Strabismic amblyopia.

2. Ocular torticollis.

Treatment:-

I. Wait for 6 months aiming for the condition to become stable. During this time we do:

1. Refer the patient to a neurologist for etiological diagnosis.

2. To relieve the patient from diplopia we do either:

- Relieving prism with its base to direction of weak muscle.

- Alternating cover (we do not cover the diseased eye for a long time for fear of diplopia).

II. After 6 months if the condition is not cured we do:

1. Relieving prisms to relief diplopia.

2. EOM surgery:

- Strengthening of the weak muscle by resection if there is considerable residual action.

- Muscle transplantation if the affected muscle is damaged to a large extent.

- Weakening of the contralateral muscle by recession.

N.B. If there is ptosis it should be treated after treatment of squint. If ptosis is corrected at first the patient will suffer from diplopia.

COVER TEST

It is a test used for diagnosis of squint. It is done as following:

- The patient fixes on a point source of light 50 cm. from him.

- Before doing the cover test we notice if there is PAD or not.

A. If there is no PAD (orthophoria or heterophoria):-

- We cover both eyes alternatingly (alternating cover test).

- The aim is to dissociate binocular vision.

- If there is no SAD ---> orthophoria (either normal or apparent squint).

- If there is SAD ---> latent squint. The direction of squint is known from the direction of recovery movement.

B. If there is PAD (paralytic or concomitant squint):-

- We cover the fixating eye for few seconds then uncover it.

- The aim is to allow fixation by the squinted eye and notice SAD.

- If SAD > PAD ---> paralytic squint.

- If SAD = PAD ---> concomitant squint. In this case we do the cover test for a second time (second cover test) to differentiate between unilateral and alternating squint. Also we cover the fixating eye, then uncover it:

It occurs due to lesions affecting the medial longitudinal bundle (MLB) in the brain stem. This bundle contains neurons that are responsible for binocular eye movements. So that these lesions will cause “gaze palsy”, i.e; both eyes are straight in the primary position without diplopia. But the patient is unable to look to one side voluntarily. Muscles are not paralyzed and can respond to vestibular reflexes as caloric stimulation.

Nystagmus

It is involuntary oscillatory movement of the eye.

Causes:-

A. Physiological nystagmus:

1 . End point nystagmus: It occurs in extreme gaze positions.

2. Opto-kinetic nystagmus (OKN): It occurs when a person is looking to objects that are moving in the same direction. The person will fix to one object so that his eyes will move slowly in its direction till the object disappears. Then the eye move jerkily into the opposite direction to fix to another object. It is used clinically to measure visual acuity in children and to detect malingers.

3. Vestibular nystagmus: It is a jerky nystagmus that occurs in response to caloric stimulation of semicircular canals. It obeys the “COWS” role, i.e; cold water causes nystagmus to the opposite direction while warm water results in nystagmus to the same direction.

B. Ocular nystagmus (sensory deprivation nystagmus):

It occurs due to poor vision in both eyes since early childhood (the 2-4-6 rule). It is pendular and horizontal.

C. Motor imbalance nystagmus:

The primary defect is in the motor pathway.

Types:-

1. Jerk:

Rapid phase to one direction and slow phase to the opposite direction. The direction of nystagmus is defined as the direction of rapid phase.

2. Pendular:

The eyes move by equal velocity in both directions.

3. Mixed:

It is pendular in the primary position and becomes jerk on looking to one side.

NB. Each type can be vertical, horizontal, rotatory or oblique.

NB. Each type can be fine, medium or course according to the amplitude of eye movement.